The present invention relates to a process for liquefying natural gas in order to produce liquefied natural gas (LNG). Still more particularly, the present invention relates to liquefying natural gas that comprises mostly methane, preferably at least 85% methane, with its other main constituents being selected from nitrogen, and C-2 to C-4 alkanes, namely ethane, propane, and butane.
The present invention also relates to a liquefaction installation located on a ship or a support floating at sea, either in open sea or in a protected zone such as a port, or indeed an installation on land for medium and large units for liquefying natural gas.
Methane-based natural gas is either a by-product of an oil field, being produced in small or medium quantities, in general in association with crude oil, or else a major product of a gas field, where it is obtained in combination with other gases, mainly C-2 to C-4 alkanes, CO2, and nitrogen.
When small quantities of natural gas are associated with crude oil, the natural gas is generally treated and separated and then used on site as fuel in turbines or piston engines for producing electricity and for producing heat used in separation or production processes.
When the quantities of natural gas are large, or indeed very large, it is desirable to transport the gas so that it can be used in far-off regions, generally on other continents, and for this purpose the preferred method is to transport it in a cryogenic liquid state (−165° C.) substantially at ambient atmospheric pressure. Specialized transport ships known as methane tankers possess tanks of very large dimensions and extreme thermal insulation so as to limit evaporation during the voyage.
Gas is generally liquefied for transport purposes in the proximity of the site where it is produced, generally on land, and that operation requires large installations for reaching capacities of several thousands of (metric) tonnes (t) per year, with the largest presently existing plants combining three or four liquefaction units capable of producing 3 megatonnes (Mt) to 4 Mt per year and per unit.
That method of liquefaction requires large quantities of mechanical energy, with that mechanical energy generally being produced on site by taking a fraction of the gas in order to produce the energy needed for the liquefaction process. A portion of the gas is then used as fuel in gas turbines, in steam boilers, or in piston combustion engines.
Multiple thermodynamic cycles have been developed for optimizing overall energy efficiency. There are two main types of cycle. A first type is based on compressing and expanding a refrigerant fluid, with a change of phase, and a second type is based on compressing and expanding a refrigerant gas without a change of phase. The term “refrigerant fluid” or “refrigerant gas” is used to designate a gas or a mixture of gases circulating in a closed circuit and being subjected to stages of compression, possibly also of liquefaction, and to exchanges of heat with the surroundings, and then to stages of expansion, possibly also of evaporation, and finally to exchanges of heat with methane-containing natural gas for liquefying, which gas cools little by little to reach its liquefaction temperature at atmospheric pressure, i.e. about −165° C. for LNG.
Said first type of cycle, with a change of phase, is generally used for installations of large production capacity requiring a larger amount of equipment. Furthermore, refrigerant fluids, which are generally in the form of mixtures, are constituted by butane, propane, ethane, and methane, which gases are dangerous since in the event of a leak they run the risk of leading to explosions or large fires. Nevertheless, in spite of the complexity of the equipment required, they remain more efficient and they consume energy of about 0.3 kilowatt hours (kWh) per kilogram (kg) of LNG produced.
Numerous variants of that first type of process with phase change of the refrigerant fluid have been developed, and the various suppliers of technology or equipment have their own formulations of mixtures for association with specific pieces of equipment, both for so-called “cascade” processes in which the various refrigerant fluids used are single-component fluids and circulate in different flow circuit loops, and for so-called “mixed” cycle processes having multicomponent refrigerant fluid loops. The complexity of installations comes from the fact that in stages in which the refrigerant fluid is in the liquid state, and more particularly in separators and in connection pipes, it is necessary to install gravity collectors, also referred to herein as “separator tanks”, for gathering together the liquid phase and sending it to the cores of heat exchangers where it then vaporizes on coming into contact with the methane for cooling and liquefying, in order to obtain LNG.
The second type of liquefaction process, i.e. a process without a change of phase in the refrigerant gas, comprises a Claude cycle or an inverse Brayton cycle using a gas such as nitrogen. That second type of process presents advantages in terms of safety since the refrigerant gas in the cycle, generally nitrogen, is inert, and therefore not combustible, and that is very advantageous when installations are concentrated in a small area, e.g. on the deck of a floating support located in open sea, where such equipment is often installed on a plurality of levels, one above the other, and on an area that is reduced to the bare minimum. Thus, in the event of refrigerant gas leaking, there is no danger of explosion and it then suffices to reinject the lost fraction of refrigerant gas into the circuit. In contrast, the efficiency of that second type is lower since it generally requires energy of the order of 0.5 kWh/kg of LNG produced, i.e. about 20.84 kW days per tonne.